Characterisation of haemolytic RTX toxins produced by Australian isolates of Actinobacillus pleuropneumoniae

Abstract

The haemolytic RTX toxins of 27 isolates of A. pleuropneumoniae , representing all serovars which have been isolated from pigs in Australia, were characterized. The quantity of protein secreted by these isolates into the media was not significantly different between serovars, but haemolytic activity was only detected in the unconcentrated supernatants from cultures of serovar 1 and 5 isolates. Haemolytic activity in supernatants of serovar 2, 3 and 7 isolates was detected only after the supernatants were concentrated. On Southern hybridization blots, genomic DNA of serovar 1 and 5 isolates contained regions which were similar to the cloned structural genes for ApxI (apxIA ) and for ApxII (apxIIA ). In contrast, genomic DNA of serovar 2, 3 and 7 isolates only contained regions similar to, if not identical with, the cloned apxIIA gene. The haemolytic activity of the culture supernatant depended on the type or composition of media and adaptability of the bacteria to in vitro cultivation. Low passage cultures of A. pleuropneumoniae , which were characterized by waxy colonies, produced significantly weaker haemolytic activity than A. pleuropneumoniae after several passages in vitro.